Heretofore, various methods have been proposed as methods for producing dichloropentafluoropropanes (HCFC-225) represented by a chemical formula C3HCl2F5. For example, a method for obtaining dichloropentafluoropropanes by contacting dichlorofluoromethane with tetrafluoroethylene in the presence of a modified aluminum chloride catalyst, has been proposed, and techniques to apply isomerization to a mixture of various isomers of dichloropentafluoropropane obtained by such a method, have been disclosed (e.g. Patent Document 1 and Non-Patent Document 1).
However, the isomerization method disclosed in Patent Document 1 is a method to increase the content ratio of 2,2-dichloro-1,1,1,3,3-pentafluoropropane (CHF2CCl2CF3: HCFC-225aa), and by this method, it was not possible to obtain, at a high content ratio, 1,1-dichloro-2,2,3,3,3-pentafluoropropane (CHCl2CF2CF3; HCFC-225ca).
Whereas, the method disclosed in Non-Patent Document 1 is a method for synthesizing HCFC-225ca by isomerizing 1,3-dichloro-1,1,2,2,3-pentafluoropropane (CHClFCF2CClF2: HCFC-225cb) by means of a partially fluorinated aluminum chloride catalyst, and by this method, the yield of HCFC-225ca was as low as about 10%, and the production efficiency was poor. It is conceivable to carry out the reaction for an extended period of time or at a high temperature in order to increase the yield of HCFC-225ca, but in the case of using a fluorinated aluminum halide catalyst, it has been found that if the conversion is increased to increase the yield of HCFC-225ca, compounds other than HCFC-225 isomers are produced in a large amount by halogen exchange with the catalyst or by a disproportionation reaction among various isomers of HCFC-225, such being problematic.
Therefore, by the conventional methods, it has been impossible to efficiently obtain HCFC-225ca which is a starting material for synthesizing 2,3,3,3-tetrafluoropropene (CF3CF═CH2; HFO-1234yf) as a new refrigerant. That is, in recent years, use of HFO-1234yf having a small ozone destruction coefficient has been studied as a new refrigerant to replace 1,1,1,2-tetrafluoroethane (HFC-134a) as a greenhouse gas, and usefulness of HCFC-225ca has been increasing as a starting material to obtain 1,1-dichloro-2,3,3,3-tetrafluoropropene (CF3CF═CCl2; CFO-1214ya) which is a raw material for synthesis of such HFO-1234yf. However, a method of increasing the content ratio of HCFC-225ca in the mixture of various isomers of HCFC-225 to obtain HCFC-225ca efficiently, has not been found.